1. Field of the Invention
The present invention relates to a termination circuit used in a transmission line constituted by a pair of signal wires for transmitting a differential signal, a vehicle-mounted control apparatus, and a vehicle-mounted communication system including the termination circuit and the vehicle-mounted control apparatus.
2. Description of Related Art
Generally, as shown in FIG. 18A, in a communication system in which a plurality of terminal apparatuses 110 are connected to a transmission line LN having a bus-like configuration or a star-like configuration, a termination circuit 100 is connected to each of the open ends of the transmission line LN to prevent signal reflection at the open ends.
It has been known to use a termination circuit which has a function of reducing emission noise, that is, noise emitted from the terminal apparatuses 110 through the transmission line LN, and reducing immunity noise, that is, noise entering the terminal apparatuses 110 through the transmission line LN.
Usually, for a single-wire transmission line, a RC termination circuit is used as disclosed, for example, in “EMC Design of Printed Wiring Board” by Mark I. Montrose, pp. 240, published Jan. 20, 2006 by Misue Co., Ltd. And, for a double-wire transmission line for transmitting a differential signal, a split termination circuit is used, which is a modified version of the RC termination circuit, as disclosed, for example, in “Transistor Technology” 1997 July issue pp. 286, published by CQ Publishing.
As shown in FIG. 19A, the RC termination circuit 101 is constituted by a resistive element R (resistor) and a capacitive element C (capacitor) connected in series between a signal wire constituting a single-wire transmission line (or one of two signal wires constituting a double-wire transmission line) and the ground. And as shown in FIG. 19B, the split termination circuit 103 is constituted by a pair of resistive elements R respectively connected to two signal wires constituting a double-wire transmission line, a capacitive element C connected between a connection node (neutral point) of these resistive elements R and the ground.
In the following explanation, the RC termination circuit 101 and the split termination circuit 103 are referred to as a conventional circuit. By using the conventional circuit 101 or 103 in a transmission line, particularly, in a double-wire transmission line, it is possible to remove common mode noise, and accordingly to improve resistivity to immunity noise, because the capacitive element C included in the conventional circuit 101 or 103 lets an AC component on the transmission line into the ground. In addition, if the waveform of a differential signal flowing on the transmission line is asymmetrical, since it is shaped to have a symmetrical shape by charge/discharge operation of the capacitor element C, emission noise can be reduced as well.
However, since the termination circuit 100 (101, 103) is located at the open end of the transmission line, the above described anti-noise effect thereof varies depending on a distance to the open end of the transmission line. Although the terminal apparatus 110 located close to the open end can fully obtain the anti-noise effect, the terminal apparatus 110 located remote from the open end cannot fully receive the anti-noise effect.
To cope with this problem, it is known to adopt the “multi-termination concept” in which all the terminal apparatuses 110 are provided with the termination circuit 100 as shown in FIG. 18B. For example, refer to Cia (CanInAutomotive) http://www.can-cia.org/ “CANphy specification” pp. 34-35.
Incidentally, to increase the anti-noise effect, it is desirable that the resistance of the resistor element R of the conventional circuit 101 or 103 is set to a small value, so that a high frequency component (noise component) can be easily bypassed to the ground through the capacitive element C.
On the other hand, to increase the signal transmission speed of the transmission line, it is desirable that the resistance of the resistor element R of the conventional circuit 101 or 103 is set to a large value to prevent the rising and falling edges of a differential signal flowing on the transmission line from attenuating or dulling, so that a high frequency component (differential mode component) of the differential signal cannot be easily bypassed to the ground through the capacitive element C.
In addition, the resistive element R of the conventional circuit 101 or 103 constitutes, together with an output impedance of a signal transmit/receive circuit (transceiver), a voltage dividing circuit that divides down an output voltage of the transceiver. Accordingly, the amplitude of a differential signal delivered to the transmission line reduces with the reduction of the resistance of the resistive element R.
The reduction of the amplitude of a differential signal not only lowers the accuracy of digitizing the differential signal at a receiving side, but also imposes a large restriction on the length and layout of the transmission line.
It should be noted that changing the resistance of the resistive element of the termination circuit impairs the original function of the termination circuit (to suppress signal reflection by its resistive element set equal to the characteristic impedance of a twist pair wire as a transmission line), and accordingly, the resistance of the resistive element of the termination circuit cannot be changed arbitrary.
In the case of adopting the multi-termination concept, since the resistive elements included in the terminal apparatuses 110 are connected in parallel with one another, the combined resistance of these resistive elements increases with the increase of the number of the terminal apparatuses 110, the problem of the reduction of the amplitude of a differential signal arises as in the case of reducing the resistance of the resistive element.
To ensure a necessary value of the amplitude of a differential signal in the case of adopting the multi-termination concept, the resistance of the resistive element has to be increased depending on the number of the terminal apparatuses 110 connected to the transmission line. However, this lowers the anti-noise effect described above.
As explained above, ensurement of a necessary value of the amplitude of a differential signal, and improvement of a signal transmission speed of a transmission line are in a trade-off relationship, and hence it is difficult to satisfy both of them.